Technological innovations toward an integrated information society have been steadily made. Rapid progress has been made in information and communication technology, and not only personal information equipment and systems, typically for multimedia, but also equipment and systems for the communication infrastructure to be built up in the near future are expected to create a next big market.
The frequency bands intended for practical use in communication systems in Japan include two quasimicrowave bands, namely the 1.9 GHz band and 2.45 GHz band, one quasimillimetric wave band, namely the 19 GHz band, and one millimetric wave band, namely the 60 GHz band. In addition, in other countries, the 900 MHz band and 5.7 GHz band have been put to practical use in wireless LANs (local area networks).
The quasimicrowave bands are assigned to personal handy phone systems (PHSs) and indoor wireless equipment in medium speed wireless LANs, and the quasimillimetric wave band and millimetric wave band to indoor wireless equipment in high speed wireless LANs. As the demand in each frequency band grows, problems such as radio interference and wrong operation, which are due to mutual electromagnetic wave interference and delay spread, and tapping may possibly become serious.
In intelligent offices, in particular, which are equipped with a large number of communication and information apparatus, radio interference and wrong operation, which are due to mutual electromagnetic wave interference and delay spread, are apt to result. Furthermore, there are utensils such as furniture made of metal, which have a number of metal surfaces capable of reflecting electromagnetic waves, causing the problem of radio wave environment deterioration.
For improving the radio wave environment, electromagnetic wave absorbers comprising an electromagnetic wave absorbing material have so far been used. Known as the electromagnetic wave absorbing material is a composite material generally composed of ferrite and a binder. Good absorption is achieved by precisely controlling, depending on the frequency intended for use, not only the magnetic and dielectric characteristics but also the thickness of the composite material in the step of processing.
Particularly when an electromagnetic wave absorbing material is to be used as a building material, it should have durability, flame resistance and workability for fitting. Therefore, a number of electromagnetic wave absorbing materials have been proposed in which the main binder material is an inorganic material rather than an organic material. A number of building materials intended for electromagnetic wave shielding, which comprise an inorganic material, have also been proposed.
Thus, Japanese Kokai Publication Sho-49-71722 discloses an electromagnetic wave absorbing material prepared by incorporating a ferrite powder with a mean particle size of not less than 60 .mu.m as a portion of the aggregate in a building material such as concrete or mortar to give an ability to absorb electromagnetic waves. In Japanese Kokai Publication Sho-53-25898, there is disclosed a method of producing electromagnetic wave absorbers which comprises incorporating magnetic dust mainly comprising ferrite into a board material such as cement, gypsum or asbestos. In Japanese Kokai Publication Hei-01-179400, there is disclosed a building material which comprises a board of gypsum admixed with, for example, not less than 50% of special conductive fiber yarn and which has antistatic and electromagnetic wave shielding functions.
In Japanese Kokai Publication Hei-04-74747, a conductive elastic mortar composition is disclosed which comprises, as main components, cement, gypsum, alumina cement, carbon fiber and a polymer admixture.
Japanese Kokai Publication Hei-06-122568 discloses an inorganic foamed material produced by allowing a hydraulic inorganic composition comprising a hydraulic inorganic substance, water and carbon fiber to foam and set.
According to these technologies, however, the electromagnetic wave absorbing materials are thick, hence poor in processability or workability for fitting. Furthermore, when such electromagnetic wave absorbing materials are used as building materials for intelligent offices or the like in which information and communication apparatus are used, their electromagnetic wave absorbing capacity is insufficient.
In Japanese Kokai Publication Hei-04-310555, gypsum moldings are disclosed which are produced by admixing hemihydrate gypsum or anhydrous gypsum containing 0.1 to 70% by weight of at least one kind of a magnetic material and a ferroelectric material with a water absorbing polymer including water within the structure thereof, followed by setting. These gypsum moldings may effectively be used as antistatic, magnetic, conductive, or ferroelectric materials.
However, this technology refers to the coercive force of said gypsum moldings and the resistance of the powder form materials but makes no specific mention of the electromagnetic wave absorbing capacity thereof. Hard ferrite, which is insufficient in electromagnetic wave absorbing capacity, is the only ferrite species used there. Furthermore, because of the use of the water absorbing polymer, there is the problem that the strength of the boards themselves decreases upon absorption of water.
In Japanese Kokai Publication Hei-06-209180, there is disclosed an electromagnetic wave absorbing inner wall material which mainly comprises gypsum, cement or calcium silicate and contains, as an electromagnetic wave loss-causing material, carbon, ferrite, a metal powder, a metal compound powder or a mixture of these. According to this technology, it is indeed possible to produce electromagnetic wave absorbing inner wall materials capable of coping with such respective bands as 70 to 400 MHz, 400 to 900 MHz, 900 MHz to 1.5 GHz and 1.5 to 3 GHz, but it is impossible to cope with such a broad band as 70 MHz to 1.5 GHz.
Japanese Kokai Publication Hei-07-202472 discloses an electromagnetic wave shielding material produced by integrally molding an electromagnetic wave reflecting material on one side of an electromagnetic wave absorbing material. According to this technology, however, since a metallic material, such as a metallic mesh material or lattice-like metallic member, is used as an electromagnetic wave absorbing material, it is necessary to integrally mold the electromagnetic wave reflector and electromagnetic wave absorber to give a shielding material. In addition, such material is heavy, hence unfavorably poor in workability for fitting when it is used as a wall, ceiling or like material.
Furthermore, such electromagnetic wave shielding material cannot equally absorb electromagnetic waves in all frequency bands remote from one another and ranging over quasimicrowave, quasimillimetric and millimetric bands, although said material has good electromagnetic wave absorbing ability in a specific frequency band. Accordingly, with the increasing use of such quasimicrowave and quasimillimetric bands as mentioned above, it is a requirement from the relevant industry that an electromagnetic wave absorbing material capable of equally absorbing electromagnetic waves in all frequency bands from quasimicrowave bands to millimetric wave bands be developed. For designing such electromagnetic wave absorbing material, it is necessary to investigate the conditions for ferrite manufacture according to the respective frequencies of the electromagnetic waves to be absorbed and produce ferrite under the conditions found suited. It is therefore difficult to realize arbitrary matched frequency characteristics.
On the other hand, Japanese Kokai Publication Hei-03-36795 discloses an electromagnetic wave absorbing material rendered capable of absorption in broad bands by controlling the spin orientation of sintered ferrite utilizing the magnetic force of the lower hard ferrite layer to thereby vary the complex permeability. For enabling electromagnetic wave absorption in broad bands by varying the complex permeability by means of magnetic force, however, matching by controlling the ferrite layer thickness is required and a powerful and heavy magnet is also required. The effect of this electromagnetic wave absorbing material in retaining the absorbing ability in broad bands is, as a matter of fact, not very marked.
In Japanese Kokai Publication Hei-08-83994, there is disclosed, as an electromagnetic wave absorbing material that can be used in broad bands, a multilayer electromagnetic wave absorbing material comprising a first layer consisting of a conductive substrate, a second layer composed of a magnetic metal oxide in fine powder form and a binder, and a third layer constituted of a magnetic metal in fine powder form and a binder, laminated together in that order.
With the electromagnetic wave absorbing material having said structure, it is possible to steadily absorb electromagnetic waves in the broad band range of 1 to 60 GHz with low reflection absorbing ability. Unlike the sintered ferrite board disclosed in Japanese Kokai Publication Hei-08-36785, said second layer, which is composed of a magnetic metal oxide in fine powder form and a binder, is characterized in that it has a high degree of freedom concerning the matching, although the absorbing capacity is generally limited. Therefore, such low-reflection material can prevent mutual interference and delay spread of electromagnetic waves in broad bands from quasimicrowave bands to quasimillimetric and millimetric bands.
Meanwhile, in recent years, the number of the so-called intelligent buildings has increased. Generally, such buildings structurally have a large number of windows and contain a large number of metallic furniture pieces and utensils, such as lockers. Therefore, for effective prevention of mutual interference and delay spread of electromagnetic waves in broad bands, it is important to fit electromagnetic wave absorbing materials on ceiling portions where wide areas can be secured. It is thus strongly required that the electromagnetic wave absorbing material should be lightweight and excellent in workability for fitting and, when used as a building material, fit well and can be used in the same manner as the conventional building materials. Said material should further have flame resistance or incombustibility, which is required of building materials in general, by way of precaution against fire.
In view of the foregoing, it is an object of the present invention to provide an electromagnetic wave absorbing material which can absorb electromagnetic waves in a narrow band or a plurality of band in frequencies from quasimicrowave bands to millimetric wave bands used for radio communication and also which is lightweight and is resistant to flame and is excellent in handling qualities and workability as a building material.